Dynamic copy window control for domain expansion reading

ABSTRACT

Method and apparatus for reading a magneto-optical domain expansion recording medium ( 10 ), wherein the size of a spatial copy window of a domain copying process is controlled by varying a predetermined reading parameter in response to a control information derived from a readout pulse. A predetermined additional pattern of change is applied to said predetermined parameter and the control information is then derived from a deviation of a clock signal. The size of the copy window can thus be controlled dynamically to obtain a robust and reliable readout process.

The present invention relates to a method and apparatus for reading adomain expansion recording medium, such as a MAMMOS (Magnetic AMplifyingMagneto-Optical System) disk, comprising a recording or storage layerand an expansion or readout layer.

In magneto-optical storage systems the minimum width of the recordedmarks is determined by the diffraction limit, that is by the NumericalAperture (NA) of the focusing lens and the laser wavelength. A reductionof the width is generally based on applying shorter wavelength lasersand higher NA focusing optics. During magneto-optical recording theminimum bit length can be reduced to below the optical diffraction limitby using Laser Pulsed Magnetic Field Modulation (LP-MFM). In LP-MFM thebit transitions are determined by the switching of the field and thetemperature gradient induced by the switching of the laser.

FIG. 3 shows a typical pattern of crescent-shaped bits or recordeddomains as formed in the recording layer by an LP-MFM recording with awidth of 0.6 μm and a thickness of 0.2 μm. For readout of the smallcrescent-shaped marks recorded in this way Magnetic Super Resolution(MSR) or Domain Expansion (DomEx) methods have been proposed. Thesetechnologies are based on recording media with several magneto-static orexchange-coupled layers.

FIG. 2 shows a typical stack of a recording or storage layer rcl and ofa readout layer rdl for such MSR media In FIG. 2, an arrow dmd indicatesthe disk moving direction. The readout layer rdl on a magneto-opticaldisk is arranged to mask adjacent bits during reading while, accordingto domain expansion, a domain d in the center of a spot m is expanded.The advantage of the domain expansion technique over MSR results in thatbits with a length below the diffraction limit can be detected with asimilar signal-to-noise ratio (SNR) as bits with a size comparable tothe diffraction-limited spot size. MAMMOS is a domain expansion methodbased on magneto-statically coupled storage and readout layers wherein amagnetic field modulation is used for expansion and collapse of theexpanded domains d in the readout layer rdl.

In the above-mentioned domain expansion techniques, like MAMMOS, awritten mark from the storage layer rcl is copied to the readout layerrdl upon laser heating and with the help of an external magnetic field.Due to the low coercivity of this readout layer, the copied mark willexpand to fill the optical spot and can be detected with a saturatedsignal level which is independent of the mark size. Reversal of theexternal magnetic field collapses the expanded domain. On the otherhand, a space in the storage layer will not be copied and no expansionwill occur. Therefore, no signal will be detected in this case.

To read out the bits or domains in the storage layer rcl, the thermalprofile of the optical spot is used. When the temperature of the readoutlayer rdl is above a predetermined threshold value the magnetic domainsare copied from the storage layer rcl to the magneto-statically coupledreadout layer rdl. This is because the stray field H_(S) from thestorage layer rcl, which is proportional to the magnetization of thislayer, increases as a function of the temperature.

FIG. 4 shows a diagram indicating a characteristic of the magnetizationM_(S) of the storage or recording layer as a function of thetemperature. According to FIG. 4, the magnetization Ms increases as afunction of the temperature for the temperature region just above acompensation temperature T_(comp). This effect results from the use of arare-earth transition metal (RE-TM) alloy which generates twocounteracting magnetizations M_(RE) (rare earth component) and M_(TM)(transition metal component) with opposite directions.

FIG. 5 shows a diagram indicating the effect of the coercivity of thereadout layer rdl as a function of the temperature. The coercivity ofthe readout layer rdl decreases as a function of the temperature in thisregion just above the compensation temperature T_(comp). Two layers witha different compensation temperature are shown by way of example in FIG.5.

By applying an external magnetic field the copied domain in the readoutlayer rdl expands to a saturated detection signal, independent of thesize of the original domain. This copying process is non-linear. Whenthe temperature is above the threshold value, magnetic domains arecoupled from the storage layer rcl to the readout layer rdl. Fortemperatures above this threshold temperature the following condition issatisfied:H _(S) +H _(ext) ≧H _(c)  (1)where H_(S) is the stray field of the storage layer rcl at the readoutlayer rdl, H_(ext) is the external applied field and H_(c) is thecoercive field of the readout layer rdl. The spatial region where thiscopying occurs is called the ‘copy window’ w. The size of this copywindow w is very critical for accurate readout. When the condition (1)is not fulfilled (copy window w=0), no copying takes place at all. Onthe other hand, an oversized copy window w will cause overlap withneighboring bits (marks) and will lead to additional ‘interferencepeaks’. The size of the copy window w depends on the exact shape of thetemperature profile (dependent, for example, on the laser power and onthe ambient temperature), the strength of the external applied magneticfield, and on material parameters that may show range variations.

On the one hand, the laser power used in the readout process should behigh enough to enable copying. On the other hand, a higher laser powerincreases the overlap of the temperature-induced coercivity profile andthe stray field profile of the bit pattern. The coercivity H_(c)decreases and the stray field increases with increasing temperature.When the overlap becomes too large, correct readout of a space is nolonger possible due to false signals generated by neighboring marks. Thedifference between this maximum allowed laser power and the minimallyrequired laser power determines the so-called power margin. This powermargin decreases with decreasing bit length. Experiments have shown thatwhen applying current methods, bit lengths of 0.10 μm can be correctlydetected at a small power margin of less than 1%. Therefore, for higherdensities the power margin remains quite small so that optical powercontrol during readout is very important.

Conventionally, that is, in phase change recording or conventionalmagneto-optic recording, the laser power during the read mode iscontrolled by a feedback loop that measures the laser output powerusing, for example a so called Forward Sense Diode (FSD). Duringwriting, an additional running optimum power control method might beapplied to provide absorption control; such a method, for example, usesthe reflected light during the writing process.

However, the accuracy of these loops is not sufficient by far for robustreadout of MAMMOS disks. For example, a change in the ambienttemperature is generally not measured by the FSD, but it influences thecopy window.

Another idea is to control the laser power by counting the number ofdetected MAMMOS pulses from a known sequence or calculating the runningdigital sum of a detected signal from a recorded DC free modulationcode. In these cases, an insufficient laser power will result in asmaller number of pulses than expected, since no copying occurs in somecases. On the other hand, an excessive laser power will give more pulsesthan expected. A disadvantage of this kind of pulse-counting controlmethods is the fact that errors must be made deliberately to obtain anerror signal.

It is an object of the present invention to provide a reading method anda reading apparatus for domain expansion readout by means of which arobust and reliable readout process can be achieved.

This object is achieved according to the present invention by providinga method as claimed in claim 1 and by providing an apparatus as claimedin claim 8.

Accordingly, a dynamic copy window control function is provided by usinginduced clock deviations as an input for a copy window control function.The accuracy of the copy window size is thus increased to improverobustness and reliability of the readout process.

The clock signal may be recovered from the readout pulse, from a wobbledgroove, or from fine embossed clock marks provided in the disk, or fromany combination thereof.

The predetermined parameter may correspond to the value of the radiationpower. Alternatively, the predetermined parameter may correspond to thestrength of the external magnetic field. In a further embodiment, thepredetermined parameter may correspond to a combination of the radiationpower value and the magnetic field strength. Now, a coarse control maybe performed based on the radiation power value while a fine control maythen be based on the external magnetic field strength. This option ispreferred in terms of stability and power consumption. Nevertheless, thereverse option is also possible. The magnetic field strength may, forexample, be varied by varying a coil current of a magnetic head of thereadout system. Of course, as mentioned above, both parameters may beused in combination, that is, to implement a combined coarse and finecontrol function.

Furthermore, the control information may be obtained from a deviation ofa maximum value of a phase error of a recovered clock signal from apredetermined set value. The predetermined additional change pattern maybe a periodic pattern of a predetermined frequency. In particular, theperiodic pattern may be a sinusoidal pattern so as to provide easylock-in detection. Alternatively, the periodic pattern may be asquare-wave pattern, preferably at half of the bit frequency or aninteger multiple of half of the bit frequency, this has the advantagethat it is easy to implement in the laser or coil driver circuitry.

The external magnetic field may be sustained by a field control meansuntil the mark region is copied and may then be reversed in response todetection of the readout pulse.

Other advantageous embodiments are defined in the dependent claims.

In the following, the present invention will be described on the basisof a preferred embodiment and with reference to the accompanyingdrawings, in which:

FIG. 1 shows a diagram of a magneto-optical disk player according to apreferred embodiment;

FIG. 2 shows a typical stack of a recording layer and of a readout layerin a magnetic super resolution (SR) medium;

FIG. 3 shows typical crescent shaped domain regions formed in thestorage layer,

FIG. 4 shows a diagram indicating a characteristic of the magnetizationof the recording layer as a function of the temperature;

FIG. 5 shows a diagram indicating a characteristic of the coercivity ofthe readout layer as a function of the temperature;

FIG. 6 is a schematic representation of the sensitivity of the copywindow size as a function of the coercivity, the external magneticfield, and the laser power;

FIG. 7 shows a diagram indicating a characteristic of the copy windowsize as a function of the temperature;

FIG. 8 shows readout signals for constant reading parameters and a smallcopy window size equal to b/2;

FIG. 9 shows readout signals for an increased copy window size;

FIG. 10 shows a block diagram of a clock recovery circuit according to apreferred embodiment; and

FIG. 11 shows a diagram indicating a characteristic of the copy windowsize and a phase error amplitude as a function of the thresholdtemperature.

A preferred embodiment will now be described on the basis of a MAMMOSdisk player as shown in FIG. 1.

FIG. 1 schematically shows the construction of the disk player accordingto a preferred embodiment. The disk player comprises an optical pick-upunit 30 having a laser light radiating section for irradiation of amagneto-optical recording medium or record carrier 10, such as amagneto-optical disk, with light that has been converted, duringrecording, into pulses with a period synchronized with code data, and amagnetic field applying section comprising a magnetic head 12 whichapplies a magnetic field in a controlled manner at the time of recordingand playback on the magneto-optical disk 10. In the optical pick-up unit30 a laser is connected to a laser driving circuit which receivesrecording and readout pulses from a recording/readout pulse adjustingunit 32 to thereby control the pulse amplitude and timing of the laserof the optical pick-up unit 30 during a recording and readout operation.The recording/readout pulse adjusting circuit 32 receives a clock signalfrom a clock generator 26 which may comprise a PLL (Phase Locked Loop)circuit.

It is to be noted that for reasons of simplicity the magnetic head 12and the optical pick-up unit 30 are shown on opposite sides of the disk10 in FIG. 1. However, according to the preferred embodiment they shouldbe arranged on the same side of the disk 10.

The magnetic head 12 is connected to a head driver unit 14 and receives,at the time of recording, code-converted data via a phase adjustingcircuit 18 from a modulator 24. The modulator 24 converts inputrecording data into a prescribed code.

At the time of playback, the head driver 14 receives a timing signal viaa playback adjusting circuit 20 from a timing circuit 34, the playbackadjusting circuit 20 generating a synchronization signal for adjustingthe thing and amplitude of pulses applied to the magnetic head 12. Thetiming circuit 34 derives its timing signal from the data readoutoperation as described later. Thus, data-dependent field switching canbe achieved. A recording/playback switch 16 is provided for switching orselecting the respective signal to be applied to the head driver 14 atthe time of recording and at the time of playback.

Furthermore, the optical pick-up unit 30 comprises a detector fordetecting laser light reflected from the disk 10 and for generating acorresponding reading signal applied to a decoder 28 which is arrangedto decode the reading signal to generate output data. Furthermore, thereading signal generated by the optical pick-up unit 30 is applied to aclock generator 26 in which a clock signal obtained from embossed clockmarks of the disk 10 is extracted or recovered, and which applies theclock signal for synchronization purposes to the recording pulseadjusting circuit 32 and the modulator 24. In particular, a data channelclock may be generated in the PLL circuit of the clock generator 26. Itis to be noted that the clock signal obtained from the clock generator26 may as well be applied to the playback adjusting circuit 20 tothereby provide a reference or fallback synchronization which maysupport the data-dependent switching or synchronization controlled bythe timing circuit 34.

In the case of data recording, the laser of the optical pick-up unit 30is modulated with a fixed frequency corresponding to the period of thedata channel clock, and the data recording area or spot of the rotatingdisk 10 is locally heated at equal distances. Additionally, the datachannel clock output by the clock generator 26 controls the modulator 24to generate a data signal with the standard clock period. The recordingdata are modulated and code-converted by the modulator 24 to obtain abinary run length information corresponding to the information of therecording data.

The structure of the magneto-optical recording medium 10 may correspondto the structure described in JP-A-2000-260079.

In the preferred embodiment shown in FIG. 1, the timing circuit 34 isprovided for applying a data-dependent timing signal to the playbackadjusting circuit 20. As an alternative, the data-dependent switching ofthe external magnetic field may as well be achieved by applying thetiming signal to the head driver 14 so as to adjust the timing or phaseof the external magnetic field. The timing information is obtained fromthe (user) data on the disk 10. To achieve this, the playback adjustingcircuit 20 or the head driver 14 is adapted to provide an externalmagnetic field which is normally in the expansion direction. When arising signal edge of a MAMMOS peak is detected by the timing circuit 34at an input line connected to the output of the optical pick-up unit 30,the timing signal is applied to the playback adjusting circuit 20 suchthat the head driver 14 is controlled to reverse the magnetic fieldafter a short time so as to collapse the expanded domain in the readoutlayer, and reset the magnetic field to the expansion direction shortlyafter that. The total time between the peak detection and the fieldreset is set by the timing circuit 34 to correspond to the sum of themaximum allowed copy window and one channel bit length on the disk 10(times the linear disk velocity).

A dynamic copy window control function according to a preferredembodiment will be described hereinafter. FIG. 6 is a schematicrepresentation of the sensitivity of the copy window size as a functionof the coercivity, the external magnetic field, and the laser power.From experiments it appeared that for robust read-out of the domains thelaser power must be controlled with an accuracy better than 0.8%. Asindicated in FIG. 6, the threshold temperature for the copying processis determined at the point where the sum of the stray field H_(S) and ofthe external field H_(ext) equals the coercivity H_(c). In the lowerpart of the diagram a temperature profile TP and an intensity profile IPof the optical spot are plotted in the tangential direction of the disktrack. Due to the movement of the disk, the temperature profile TP hasan asymmetrical shape and the intensity profile IP slightly advances thetemperature profile in the disk movement direction. The size of the copywindow w is then determined by the width of the temperature profile TPat the threshold temperature T_(threshold), as indicated by the greyrectangular area in the lower left part of FIG. 6.

As a first-order model, the top of the temperature profile TP can beregarded as a parabola (note that only the top of the temperatureprofile is used to achieve the required high resolution read-out). Thiscan be expressed as follows:T(x)=ax ² +bx+cThe width of the copy window is now a square-root function depending onthe threshold temperature T_(threshold), as expressed by the followingequation:$w_{x} = \frac{\sqrt{b^{2} - {4a\quad c} + {4a\quad T_{threshold}}}}{a}$

The onset temperature for the copy window to occur is now:$T_{threshold} = {c - \frac{b^{2}}{4a}}$

The above function w_(x)(T) is schematically represented in FIG. 7. Thehatched region is the proper working range for MAMMOS readout where nointerference peaks will occur, if the system is properly synchronized.As can be gathered from FIG. 7, the size w of the copy window increasesaccording to a square-root function, while the amount of change of thecopy window size w, i.e. the tangential slope or derivative of thegraph, depends on the actual threshold temperature. This fact can beused to provide a copy window control functionality as follows.

The solution proposed here is to measure the size w of the copy windowcontinuously by using information from the detected data signal in theread mode. FIG. 8 shows some key signals for readout of MAMMOS disks ina steady-state situation with constant laser power, constant ambienttemperature, homogeneous disk properties, constant field strength,constant coil-disk distance, etc. The top graph shows the magnetic bitsin the storage layer. The second graph shows the overlap signal(convolution) of the magnetic bit pattern and the copy window. The thirdgraph shows the external magnetic field, and the bottom graph shows theresultant MAMMOS signal. When the overlap signal is non-zero, copying ofdomains will take place. As already mentioned, the external magneticfield is kept high until a bit or domain is copied from the storagelayer and expanded in the readout layer. Then, after a fixed delay, theexternal field is reversed and the domain is collapsed until the nextbit transition or domain copying occurs.

FIG. 9 shows a diagram similar to FIG. 8, but now one of the parametersto be controlled, e.g. the laser power, is increased deliberately. Thisincrease/decrease (wobbling) is done with a predefined change pattern,e.g. a periodic pattern with a small amplitude. The wobbling causes thecopy window to increase or decrease in size in synchronism with thewobble frequency. Comparing FIGS. 8 and 9, it becomes clear that whenthe copy window increases in size, the next transition will appearsomewhat earlier than expected. On the other hand, when the copy windowdecreases in size, the next transition will be delayed slightly. This isthe phase error Δφ shown in FIG. 9. However, as can be gathered fromFIG. 7, the increase or decrease of the copy window size, and hence thevalue of the phase error Δφ, depends on the actual thresholdtemperature.

When the wobble frequency lies above the bandwidth of the clock recoveryPLL circuit of the clock generator 26, the phase error of this PLLcircuit can be used to detect the small deviation or phase error Δφ fromthe expected transition position. The average value of the frequencydeviation of the introduced wobble or change pattern should be zero.

FIG. 10 shows an example of the PLL circuit of the clock generator 26according to the preferred embodiment. The detected run length signaloutput from the pickup unit 30 is applied to a phase detector 261 inwhich the phase of the run length signal is compared with the phase of afeedback signal obtained from a clock divider 265 to which the outputsignal of a voltage-controlled oscillator (VCO) 264 is applied. Theoutput of the phase detector 261, corresponding to the phase differencebetween the run length signal and the feedback signal, is applied to aloop filter 263 for extracting the desired frequency to bephase-controlled in the PLL circuit. A band-pass filter 262 with acenter frequency around the wobble frequency can be used for low-noisedetection of the phase error Δφ, i.e. lock-in detection. The phase errorΔφ obtained here cannot be used yet as an absolute error signal forlaser power control as only the absolute scale is known, but noreference (zero or offset) is present. This means that only changes inthe size of the copy window can be measured.

To circumvent this problem, the derivative of the copy window size w afunction of temperature can be measured to obtain a control informationfor controlling the size of the copy window.

FIG. 11 shows the derivative of the copy window characteristic of FIG.7. Due to the fact that the derivative or amount of change of the copywindow size directly leads to the phase error Δφ, the amplitude of thedetected phase error Δφ corresponds to the derivative and hence can beused for copy window control. As a reference condition, this amplitudeof the phase error Δφ must satisfy an initially determined set conditionor setpoint sp. The deviation from this setpoint sp can then be used asa control signal PE for the laser power control procedure or forcontrolling any other suitable reading parameter, e.g. strength of theexternal magnetic field.

Any changes in the size w of the copy window due to changes inparameters, such as coil-disk distance, ambient temperature, etc., arecounteracted by the controlled parameter, e.g. laser power in thepresent example.

However, when the laser power is controlled, the system might sufferfrom a slight thermal memory that causes a phase shift of the appliedwobble signal. In principle, this shift can be compensated for byintroducing a delay in the control loop, which should be a function ofthe disk velocity and also depends on the disk stack (thermal design).As an alternative, the field strength of the external magnetic fieldmight be used as a control parameter, e.g. by varying the coil current.It is to be noted that this control is equivalent to that given by therelation as depicted in FIG. 6, since the threshold temperature is alsoshifted in response to the strength of the external magnetic fieldH_(ext). The described idea will not change significantly in that case.

It is to be noted that the present invention can be applied to anyreading system for domain expansion magneto-optical disk storagesystems. Any suitable reading parameter can be varied to control thecopy window size. Furthermore, any suitable change pattern can beapplied to the selected reading parameter so as to induce the phaseerror of the readout signal. The preferred embodiment may thus varywithin the scope of the attached claims.

1. A reading method for reading a magneto-optical recording medium,comprising a storage layer and a readout layer, wherein an expandeddomain leading to a readout pulse is generated in said readout layer bycopying a mark region from said storage layer to said readout layer uponheating by a radiation power and with the aid of an external magneticfield, said method comprising the steps of: a) controlling the size of aspatial copy window of said copying process by varying a predeterminedreading parameter in response to a control information derived from saidreadout pulse, b) applying a predetermined additional pattern of changeto said predetermined parameter, and c) obtaining said controlinformation from a deviation of a clock signal.
 2. A method according toclaim 1, wherein said clock signal is recovered from said readout pulse,from a wobbled groove, or from embossed marks provided on said recordingmedium, or from any combination thereof.
 3. A method according to claim1, wherein said predetermined parameter corresponds to the value of saidradiation power.
 4. A method according to claim 1, wherein saidpredetermined parameter corresponds to the strength of said externalmagnetic field.
 5. A method according to claim 1, wherein saidpredetermined parameter corresponds to a combination of the value ofsaid radiation power and the strength of said external magnetic field.6. A method according to claim 5, wherein one of said values of saidradiation power and said strength of said external magnetic field isused for coarse control and the other one is used for fine control.
 7. Amethod according to claim 4, wherein said strength of said externalmagnetic field is varied by varying a coil current of a magnetic head.8. A method according to claim 1, wherein said control information isobtained from a deviation of a maximum value of a phase error of saidrecovered clock signal from a predetermined set value.
 9. A methodaccording to claim 1, wherein said predetermined additional changepattern is a periodic pattern of a predetermined frequency.
 10. A methodaccording to claim 9, wherein said periodic pattern is a sinusoidalpattern.
 11. A method according to claim 9, wherein said periodicpattern is a square-wave pattern.
 12. A method according to claim 11,wherein the frequency of said square-wave pattern corresponds to half ofa bit frequency or an integer multiple of half of the bit frequency. 13.A method according to claim 1, wherein said clock signal is recovered byusing a phase-locked loop function.
 14. A reading apparatus for readingfrom a magneto-optical recording medium comprising a storage layer and areadout layer, wherein an expanded domain leading to a readout pulse isgenerated in said readout layer by copying a mark region from saidstorage layer to said readout layer upon heating by a radiation powerand the aid of an external magnetic field, said apparatus comprising: a)control means for controlling the size of a spatial copy window of saidcopying process by varying a predetermined reading parameter in responseto a control information derived from said readout pulse, b) changemeans for applying a predetermined additional pattern of change to saidpredetermined parameter, and c) clock recovery means for obtaining saidinformation from a deviation of a clock signal.
 15. A reading apparatusaccording to claim 14, wherein said clock recovery means is arranged torecover said clock signal from said readout pulse, from a wobbledgroove, or from embossed marks provided on said recording medium, orfrom any combination thereof.
 16. A reading apparatus according to claim14, wherein said control means is arranged to vary said radiation power.17. A reading apparatus according to claim 14, wherein said controlmeans is arranged to vary said external magnetic field.
 18. A readingapparatus according to claim 14, wherein said control means is arrangedto vary the value of said radiation power and the strength of saidexternal magnetic field in combination.
 19. A reading apparatusaccording to claim 18, wherein said control means is arranged to use oneof said values of said radiation power and said strength of saidexternal magnetic field for coarse control and the other one for finecontrol.
 20. A reading apparatus according to claim 14, also comprisingfield control means for sustaining said external magnetic field untilsaid mark region is copied and for reversing said external magneticfield in response to detection of said readout pulse.
 21. A readingapparatus according to claim 14, wherein said clock recovery means isarranged to obtain said control information from a deviation of amaximum value of a phase error of said clock signal from a predeterminedset value.
 22. A reading apparatus according to claim 14, wherein saidclock recovery means comprises a phase-locked loop circuit.
 23. Areading apparatus according to claim 14, wherein said change means isarranged to use a periodic pattern of a predetermined frequency as saidpredetermined additional change pattern.
 24. A reading apparatusaccording to claim 23, wherein said periodic pattern is a sinusoidalpattern.
 25. A reading apparatus according to claim 23, wherein saidperiodic pattern is a square-wave pattern.
 26. A reading apparatusaccording to claim 25, wherein the frequency of said square-wave patterncorresponds to half of a bit frequency or an integer multiple of half ofthe bit frequency.
 27. A reading apparatus according to claim 14,wherein said reading apparatus is a disk player for MAMMOS disks.